Reductive Sn2+ Compensator for Efficient and Stable Sn‐Pb Mixed Perovskite Solar Cells

Abstract Tin‐lead (Sn‐Pb) mixed perovskite with a narrow bandgap is an ideal candidate for single‐junction solar cells approaching the Shockley‐Queisser limit. However, due to the easy oxidation of Sn2+, the efficiency and stability of Sn‐Pb mixed perovskite solar cells (PSCs) still lag far behind that of Pb‐based solar cells. Herein, highly efficient and stable FA0.5MA0.5Pb0.5Sn0.5I0.47Br0.03 compositional PSCs are achieved by introducing an appropriate amount of multifunctional Tin (II) oxalate (SnC2O4). SnC2O4 with compensative Sn2+ and reductive oxalate group C2O4 2− effectively passivates the cation and anion defects simultaneously, thereby leading to more n‐type perovskite films. Benefitting from the energy level alignment and the suppression of bulk nonradiative recombination, the Sn‐Pb mixed perovskite solar cell treated with SnC2O4 achieves a power conversion efficiency of 21.43%. More importantly, chemically reductive C2O4 2− effectively suppresses the notorious oxidation of Sn2+, leading to significant enhancement in stability. Particularly, it dramatically improves light stability.

Device fabrication: Indium doped tin oxide (ITO) glass substrate is ultrasonically cleaned with deionized water and alcohol, respectively, for 15 minutes at a time.
PEDOT: PSS aqueous solution films are coated on the cleaned ITO substrate at 4000 rpm for the 30s and then dried at 150 °C for 15 min in ambient air.The substrates are then transferred into a N2-filled glovebox for the deposition of perovskite films.The 1.6M FA0.5MA0.5Sn0.5Pb0.5I0.47Br0.03solution is spin-coated onto the PEDOT: PSS substrate at 4000 rpm for 30 s, and CB (180 µl) is dropped onto the substrate at 20 s and then dried at 100 ℃ for 5 min.Subsequently, the PCBM (40µl, 10 mg mL −1 ) solution in chlorobenzene is deposited on perovskite film at 2500 rpm for 30 s and then dried at 60 ℃ for 10 min.Then the samples are put into a high vacuum chamber for the following deposition of functional layers.23 nm C60 and 7 nm BCP are sequentially evaporated onto the perovskite.70 nm Ag is finally deposited.

Characterizations:
Atomic force microscopy (AFM) is conducted on Veeco (America) and Kelvin probe force microscopy (KPFM) is used to detect the contact potential to reveal the fermi level and ion migration properties of the film under the bias applied normally to the film.The surface morphology of the films and the cross-section morphology of the devices are tested by a Hitachi SU-70 scanning electron microscope (SEM).X-ray diffraction (XRD) patterns of films are tested by a Bruker D8 advanced instrument and using Cu Kα as radiation (λ=6.162Å) at a scan rate of 4 º min -1 and diffraction angle range from 10 º to 40 º.The absorption spectra of the film on the glass substrate are measured by a UV-visible spectrophotometer (Agilent, USA).
FTIR spectroscopy is taken with an FTIR spectrometer instrument (Thermo, Nicolet 6700).The binding energies of the elements in PVK are tested by X-ray photoelectron spectroscopy (XPS, Shimadzu, Japan) using Al Kα radiation.Au is used to calibrate the energy state of the spectroscopy before measurement.The ultraviolet photoelectron spectroscopy (UPS) pattern is detected by the Axis Ultra DLD, and using the He I (21.22 eV) emission line.The steady-state fluorescence (PL) spectra of the films are measured based on the glass by a fluorescence spectrophotometer (Agilent, USA), with an excitation wavelength of 532 nm.
The SCLC measurements and dark J-V curves are measured by the Keithley 4200 under dark conditions.The J-V characteristic curves are tested by the Keithley 4200 meter and the sunlight simulator (Newport, 91192A, AM 1.5, 1 sun) whose light intensity is calibrated by a standard silicon solar cell.The solar cells are masked with a black aperture to define the active area of 0.08 cm 2 .The light intensity is calibrated to AM 1.5G (100 mWcm -2 ) by using a reference Si solar cell.The external quantum efficiency (EQE) spectra are recorded with the Newport EQE system in which the light intensity at every wavelength is calibrated with a Si detector before measurement and the wavelength range from 300 to 1100 nm.The transient photo-current (TPC) decay and transient photo-voltage (TPV) decay are recorded by an electrochemical workstation (Zahner, Germany) with a white light LED supplied 80 mWcm -2 light intensity to excite the perovskite solar cells.Electrochemical impedance spectroscopy (EIS) is performed in the frequency range from 10 Hz to 1 MHz by an electrochemical workstation in dark conditions with a bias of 0.6 V.

Figure S1
Figure S1 Digital photos of mixed Pb-Sn perovskite precursor solutions, showing the oxidation of Sn 2+ (yellow in solution) to Sn 4+ (red in solution) and the reduction of Sn 4+ to Sn 2+ by SnC2O4.During exposure to air, the vials were kept open (uncapped).

Figure S5 .
Figure S5.XRD spectra of perovskite film without or with distinct SnC2O4 doping rates.

Figure S11 .
Figure S11.Surface potential mapping at the same topography area of (a) control (b) 4% SnC2O4, and (c) their contact potential difference (CPD) line profiles.

Figure S14 .
Figure S14.Forward and reverse J-V curves s of the Champion Cell.

Figure S15 .
Figure S15.Space charge limited current measurement of the control and 4% SnC2O4 treated device.

Figure S16 .
Figure S16.Light intensity dependence of JSC for devices based on different perovskite films.

Figure S17 .
Figure S17.The dark J-V curves of devices based on control and 4% SnC2O4 devices.

Table S1 .
Detailed parameters of the energy band structure.